1. Field of the Invention
The present invention relates to a vehicle alarm system for informing other vehicles of its own presence, and more specifically to a vehicle alarm system for transmitting an alarm signal from an emergency car, for instance to other vehicles running ahead along or across the road along which the emergency car itself is travelling.
2. Description of the Prior Art
Conventionally, emergency vehicles (e.g. ambulance car, fire truck, etc.) travel by sounding a siren or blinking an alarm light. Recently, however, since almost all vehicles are running under the conditions that the vehicle windows are kept closed and the car's interior cooling apparatus is kept turned on, vehicle drivers generally have difficulty noticing the approach of an emergency vehicle, before the emergency vehicle comes close by their own vehicles. To overcome the above-mentioned problem, an alarm system for an automotive vehicle for transmitting an alarm radio wave to other vehicles (in addition to the siren and light) has been proposed.
An example of prior-art alarm systems of this type is disclosed in Japanese Published Unexamined (Kokai) Pat. Appli. No. 55-156731, as shown in FIGS. 1A and 1B. In FIG. 1B, an automotive vehicle 401 is provided with transmitting means 502 and a transmitting antenna 501. Further, two automotive vehicles 402 and 403 are provided with receiving means 504, a receiving antenna 503, and alarming means 505, respectively. In operation of this prior-art system, when the vehicle 401 transmits a radio wave from the transmitting means 502 via the transmitting antenna 501 toward the travelling direction, the vehicles 402 and 403 receive the radio wave transmitted from the vehicle 401 by the receiving means 504 via the receiving antenna 503, so that the alarming means generates an alarm to the respective drivers.
Therefore, as shown in FIG. 1A, even if the driver of the vehicle 402 cannot directly see the vehicle 401 due to the presence of a building 405, for instance, the driver can know that the vehicle 401 travelling on another road approaches an intersection.
In the prior-art alarm system as described above, however, whenever a vehicle transmits a radio wave, since the transmitted wave is received by all the vehicles running within a wave receivable area, the alarm is given to all the vehicle drivers, thus resulting in the following problem: For instance, in FIG. 1A, if the vehicle 401 travelling toward an intersection transmits a radio wave, the transmitted radio wave is received by both the vehicles 402 and 403 and the received alarm is given to both the drivers. In this case, the driver of the vehicle 403 cannot discriminate whether the alarm is transmitted from the opposing vehicle 401 travelling on the same road or the vehicle 402 travelling on a road crossing the same road. In other words, where there exists an opposing vehicle, the driver cannot discriminate whether the current alarm is transmitted from the opposing vehicle or another vehicle.